Conductive thermoplastic resin compositions utilizing a mixture of a thermoplastic resin with a conductive material have been used to form various articles using injection molding or injection-compression molding processes. These compositions and processes offer advantages such as low cost and/or the ability to mass-produce these articles.
Further, conductive thermoplastic resins have also been used as a metal substitute in lieu of fabricated metal articles and conductive surface-fabricated articles treated with a conductive coating material or metal plating. However, these conductive thermoplastic resins often had issues of low mechanical strength, thereby limiting their use despite their lower cost and ease of manufacture.
The prior art attempted to remedy these issues and improve the strength of conductive thermoplastic resins through the addition of fibrous reinforcing materials such as glass fiber. However, obtaining single conductive pellets of long glass fiber-reinforced thermoplastic resin is difficult. Pellets of long glass fiber-reinforced thermoplastic are generally obtained via pultrusion. Pultrusion is a process for manufacturing composite materials in which the reinforcing fibers are pulled through a resin and into a heated die and then pelletized to form the pellets. However, conductive fillers, such as carbon black, are often detrimental to mechanical properties despite the reinforcing fiber. In addition, the conductive filler interferes, during the pultrusion process, with impregnation of the glass fiber, causing further degradation of the mechanical properties.
To avoid this issue, most long fiber reinforced materials incorporate the additive into a separate pellet that is then dry-blended with the fiber-reinforced pellet to form a mix that is then used in the injection molding process. However, these dry blends present several problems. First producing two kinds of pellets is more expensive and complex. Second, it is likely that the kinds of pellets have different density, in particular the pellets containing long glass fibers are likely to be have higher density than the pellets containing the conductive filler, and therefore it is likely that de-mixing of the different kinds of pellets will occur during storage or handling. As a result, the final article may often have inhomogeneous properties.
Accordingly, it would be beneficial to provide a single pellet solution that eliminates the disadvantages of dry blends to form long fiber reinforced conductive articles. It would also be beneficial to provide a single pellet solution that helps reduce fiber breakage thereby enabling the advantages of the long fibers in the long fiber reinforced conductive articles to be maintained. It would also be beneficial to provide a method of making a single pellet incorporating a conductive filler and long fibers such that the resulting molded article using these pellets has enhanced properties as compared to molded articles manufactured using a dry blend.